In stepper motors, it is common to select a speed profile for the motor, said speed profile being achieved by means of the sequence of the energization periods of the motor windings.
Stepper motor drives have the general problem that they accelerate and decelerate not only the actual load, but in addition also parasitic moments of inertia, so that the stepper motor produces a significantly higher torque than would be needed for the purely static load, and particularly so during the acceleration phase.
Furthermore, there are applications where the load torque can vary greatly in a temporally indefinite manner.
In such cases, it is advantageous for the current in the windings of the stepper motor to automatically adjust itself to the particular prevailing load conditions.
East German patent document DD 277 570 A1 describes a circuit arrangement for a self-timed stepper motor, said circuit arrangement controlling the current flow time and the current amplitude in the motor windings automatically depending on load.
To this end, the encoder signals controlling the motor, and the phase signals controlling the commutation of the windings, are used to derive a signal which is representative of the load phase angle between the actual position of the pole of the stepper motor and the controlling magnetic field and is used to control the current amplitude and the current flow time.
In the process, the current-controlling signal (Ua) switches to HIGH when an encoder signal changes, whereas when the phase change derived from the change of the encoder signal is output, said current-controlling signal switches back to LOW.
When a change of an encoder signal occurs, then the poles of the motor are already in a position where the next reversal of the winding energization could be triggered. If the stepper motor runs at a predetermined stepping rate and is not overloaded, then the change of the encoder signals occurs before the associated winding changes are output, so that the current-controlling signal (Ua) has a high phase. In this connection, East German patent document DD 277 570 A1 describes a circuit arrangement in which, during the high time of the current-controlling signal (Ua), the current supply is simultaneously interrupted in both motor windings, while, in parallel, an integration element (R4, C1) performs an integration over the duty factor of the current-controlling signal (Ua). If the duty factor increases, the output voltage of integration element (Uc) reduces the current amplitude in both motor windings simultaneously for the next motor steps.
This method has the disadvantage that there is only one current-controlling signal for both motor windings.
It is also a disadvantage that the current supply occurs in both motor windings simultaneously as a function of the high phase of the current-controlling signal, and also that the current amplitude is controlled using an integration element having a finite time constant. As a result, the control characteristics are relatively slow and the rotor may oscillate with respect to the rotating magnetic field.
Another disadvantage is that, in order to avoid oscillations, the integration time constant is preferably defined anew for each drive and each type of loading.
FIG. 1 illustrates the circuit arrangement according to DD 277 570 A1, which satisfies these requirements at least for low stepping frequencies, but which requires motor- and load-adapted dimensioning of the integration components, so as to prevent the rotor from oscillating with respect to the magnetic field.
FIG. 2 shows the signal patterns occurring in the circuit arrangement of FIG. 1.
It can be seen from row Ua in FIG. 2 that the signal (Ua) produced at the collector of transistor (Tr) switches to HIGH when a change of an encoder signal (ENCA; ENCB) occurs, and that signal (Ua) switches back to LOW when there is a change of the phase signals (PHA; PHB), which is associated with the encoder change.
In this connection, signal Ua switches to HIGH prior to a change of signals (PHA; PHB). During the interval in which current-controlling signal Ua is HIGH, the torque component of the winding in which the next reversal of the current direction will occur is small, and may already be negative.
Conversely, the motor winding which is not switched over after the HIGH time of Ua has elapsed produces a large torque component.
In the circuit of FIG. 1, the reduction in current supply occurs in both motor windings simultaneously, so that the maximum torque component in the motor winding in which the current direction is not reversed after signal Ua is not produced again until end of the current risetime.